Endocrinology

• Endocrine system


  Comprises ductless glands, which empty their contents directly into the bloodstream
• Hormones


  Substances secreted by endocrine glands that have either a stimulating or inhibitory effect on development or functioning of body organs and structures
• Glands of the endocrine system

  • Hypothalamus
  • Pituitary (Anterior and Posterior)
  • Pineal gland
  • Thyroid gland
  • Parathyroid gland
  • Thymus
  • Stomach
  • Pancreas
  • Small intestines
  • Adrenals
  • Kidneys
  • Gonads (Ovary/Testis)
  • Uterus
  • Placenta

  
• Hypothalamus


  • Control center for internal environment
  • Regulates nervous and endocrine systems via 3 mechanisms: ANS centers exert nervous control on adrenal medulla, ADH and Oxytocin production, Regulatory hormone production (RH and IH) controls pituitary gland directly and all other endocrine glands indirectly
• Thyroid releasing factor (TRH)


  Stimulates the adenohypophysis to release thyrotropin
• Somatostatin


  Secreted by delta cells in digestive system and neuroendocrine neurons in hypothalamus, inhibits secretion of growth hormone from anterior pituitary
• Pituitary (hypophysis)


  Found at the base of the brain, has an anterior part (adenohypophysis) and a posterior part (neurohypophysis)

  
• Parts of the anterior pituitary (adenohypophysis)

  • Pars tuberalis
  • Pars intermedia
  • Pars distalis

  
• Acidophils and basophils


  Chromophil cells in the adenohypophysis that take up stain and are divided into different classes with different secretory products and target organs
• Hormones secreted by the anterior pituitary

  • Growth hormone (GH)
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Prolactin
  • Thyroid stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
• Posterior pituitary (neurohypophysis)


  Contains non-myelinated axons which are the neurosecretory cells, secretes antidiuretic hormone (ADH) and oxytocin

  
• Pineal gland


  Produces melatonin, which regulates daily body rhythms and inhibits secretion of gonadotropins before puberty
• The pineal gland contains high levels of fluoride which can inhibit enzymes and reduce melatonin levels
• Thyroid gland


  Composed of follicles filled with colloid, secretes thyroxine (T4) and triiodothyronine (T3) which regulate basal metabolic rate
• Parathyroid gland


  Contains chief cells that secrete parathyroid hormone (PTH) which increases blood calcium levels
• Parathyroid hormone acts on osteoclasts and renal tubular epithelial cells to increase plasma calcium
• Parathyroid gland


  • Contains oxyphil cells, dark chief cells, light chief cells and the syncytial cells

  
• Parathormone


  Hormone produced by the parathyroid gland that increases blood calcium levels
• The parathyroid gland is embedded in the capsule of the thyroid gland
• Chief cells


  • Small and pale eosinophilic staining, secrete parathyroid hormone (PTH), resting cells have paler cytoplasm than active cells
• In adults about 80% of the parathyroid cells are resting, in children more cells are active
• PTH


  Acts on osteoclasts and renal tubule epithelial cells to increase plasma calcium by promoting bone resorption and increasing renal calcium resorption
• Parathyroid gland cell types

  • Chief (or principal) cells
  • Oxyphil cells

  
• Chief (or principal) cells


  Secrete parathyroid hormone (PTH), main function is to raise serum calcium and lower serum phosphate, main control is serum calcium level
• Oxyphil cells


  Occasional cells or small clusters, function unknown, name means "acid [stain] loving" (Greek)
• The thymus is a triangular or Y-shaped lymphoid tissue which may extend from the base of the heart along the trachea to the larynx

  
• The thymus diminishes in size in adults and may disappear altogether
• Thymus histology


  • Central medulla and peripheral cortex, contains thymic stromal cells and cells of hematopoietic origin (developing T-cells are referred to as thymocytes)

  
• Peculiar nest-like bodies, the concentric corpuscles of Hassall, occur in the thymus medulla
• Concentric corpuscles of Hassall

  Composed of a central mass, consisting of one or more granular cells, and a capsule formed of epithelioid cells
• Main function of the thymus

  Production of thymic (T) lymphocytes, which migrate to other lymphoid organs developing the body's immune system
• The thymus is also thought to produce some thymosin which stimulates the development of precursor T cells in the thymus to mature T cells
• Endocrine cells (argentaffin or enterochromaffin cells) in the oxyntic glands of gastric pits of the stomach wall


  Produce gastrin, a hormone that stimulates the stomach to produce digestive juices
• Cell types in the stomach

  • O-oxyntic cells (secrete HCL)
  • E-Enterochromaffin cells (secrete gastrin)
  • C-chief cells (secrete pepsinogen)
• The endocrine pancreas contains islets of Langerhans which appear as circular or oval areas containing pale staining cells

  
• Islets of Langerhans


  • Associated with numerous small blood vessels and capillaries, contain polyhedral cells including alpha, beta, delta and PP cells
• Alpha cells


  Occupy the central part of the islets, form 20% of the islet cells, secrete glucagon which increases blood glucose levels
• Beta cells


  Occupy a peripheral position in the islets, constitute 75%-80% of the islet cells, secrete insulin which allows glucose utilization by cells
• Delta cells


  Occupy a central position in the islets, form only about 5% of the islet cell population, thought to secrete serotonin or pancreatic gastrin
• Pancreatic polypeptide (PP) cells


  Very rare cells in humans, secrete pancreatic polypeptide which stimulates the chief cells of the stomach to produce more pepsinogen